In any sort of data transmission, especially high speed data transmission, from a source, through a line to a receiver (or load), there is often an impedance mismatch. This occurs when the impedance of the source ZS, the impedance of the line ZO and the impedance of the receiver ZL are not equal. This means that, at each boundary between one impedance and another, there will not be perfect transmission of the signal but there will be partial reflection.
This impedance mismatch can cause degradation of the transmitted signal, for example by overshoot, undershoot, ringing and stair step wave forms. Those waveform distortions are shown in FIG. 1.
In an attempt to deal with this problem, in printed circuit board (PCB) design, care is taken to match the trace impedance of the PCB (i.e. the characteristic impedance of the traces, or wires, on a PCB) with the source impedance ZS, since the PCB trace impedance acts as the line impedance ZO. However, due to variations in PCB geometry and operating conditions, the impedance will not, in fact, be consistent over several PCB boards or over different operating temperatures. Errors in signalling, caused, for example, by the types of signal degradation mentioned above, result.
An attempt has been made to solve this problem by adding an additional series resistor at the source, in order to match the source impedance and the line impedance (PCB trace) more accurately. The value of the series resistor can be chosen appropriately. Although this may work in some conditions, the series resistor typically has a tolerance of around 1% and this, coupled with a typical PCB tolerance of around 10%, can result in an 11% difference between the best matched case and the worst matched case This means inconsistent product design and yield loss.
An improvement on that arrangement has been suggested and comprises making the source series resistor programmable by a user. In that known arrangement, the series resistor may take one of several values and can be set to the most appropriate value by the user, according to operating conditions and the PCB trace impedance. This does improve the source to line impedance matching, but it does not remove signal degradation completely.